1. Field of the Invention
The present invention relates to a sensor used for non-destructive measurement of electrical characteristics of semiconductor wafers, and to a method of manufacturing the same.
2. Description of the Related Art
C--V curve measurement is one of the well-known methods used for evaluating the characteristics of a metal-insulator-semiconductor (MIS) diode.
In conventional C--V curve measurement, an electrode is formed on a surface of the oxide film on a semiconductor wafer. Formation of such an electrode, however, affects electrical characteristics of the semiconductor wafer and furthermore takes much labor and time. The inventors has therefore developed a novel non-destructive measuring apparatus for evaluating the electrical characteristics of a semiconductor wafer including a C--V curve and a C--t curve without forming any electrode on a surface of the semiconductor wafer.
FIGS. 1(a) through 1(c) show principles of a non-destructive measuring apparatus for semiconductors developed by the inventors. As shown in FIG. 1(a), an oxide film 102 is formed on a surface of a semiconductor substrate 101, and an electrode 202 is formed on the other surface of the semiconductor substrate 101. Another electrode 201 is held apart from the oxide film 102 across a gap Ge by an electrode holder unit 300. The electrode holder unit 300 adjusts the gap Ge to be no more than about 1 .mu.m. The detailed structure of the non-destructive measuring apparatus and the method of measuring the gap are described in U.S. Pat. No. 5,233,291, the disclosure of which is incorporated by reference herein.
FIG. 1(b) shows an equivalent circuit of the structure shown in FIG. 1(a). The equivalent circuit between the electrodes 201 and 202 includes a series combination of a substrate capacitance Cs, an oxide capacitance Ci, and a gap capacitance Cg. The C--V curve is voltage dependency of a semiconductor capacitance Cta, which is a composite capacitance of the series combination of the capacitances Cs and Ci. What is measured by a measurement unit 400, however, is not the semiconductor capacitance Cta but the total capacitance Ct between the electrodes 201 and 202. The gap Ge can be precisely measured by the electrode holder unit 300, and the electrostatic capacitance Cg is determined according to the measured value of the gap Ge. The semiconductor capacitance Cta is then obtained by subtracting the gap capacitance Cg from the total capacitance Ct measured by the measurement unit 400.
The precision of the gap measurement and that of the electrical characteristics measurement depends upon the structure of the sensor having the measuring electrode 201. However, since there has been no apparatus Which perform non-destructive measurement of the electrical characteristics of semiconductors, no sensors suitable for such apparatus have been known.
The structure of the sensor is required to be simple enough to form a fine electrode pattern on a surface of the sensor. The dielectric breakdown strength of the sensor is required to be considerably high because the gap between the sensor and the semiconductor wafer is kept no more than about 1 .mu.m and because the electric field across the gap is to be very strong accordingly.